Reversing Sensor Problem

I have a Prius T-Spirit, registered in London, UK, in Augst 2005.
The reversing sensors normally work OK, but I find that in very hot weather, or in direct sunlight, they tell me I am close to an object all the time - i.e. there is the continuous whistle, even though I may be far away from any obstacle. This fault has always been there, but is not observed most of the time - only in high summer in England!

Has anyone else observed this, or found a solution?

(The garage, Jemca Toyota Edgware Rd. London, tell me they have not met this problem.)